Editor: Update single default template description.

In order to add more template types we need to update some copy from the current default templates provided.

This updates the `single` template copy as it's needed to clear up the ambiguity between the single template that is the fallback for any custom post type (including `post`) and the menu item to be added in a subsequent commit, that will allow the creation the `single-post` template and any `single_$post_type` template.

This commit backports the original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/41302 #41302: Site Editor: Update single default template copy]

Follow-up to [52062], [52331].

Props ntsekouras, bernhard-reiter, jameskoster, Mamaduka, scruffian.
Fixes #56534.
Built from https://develop.svn.wordpress.org/trunk@54104


git-svn-id: http://core.svn.wordpress.org/trunk@53663 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/block-template-utils.php

@@ -130,8 +130,8 @@ function get_default_block_template_types() {
 			'description' => __( 'Displays a single post or page.' ),
 		),
 		'single'         => array(
-			'title'       => _x( 'Single Post', 'Template name' ),
-			'description' => __( 'Displays a single post.' ),
+			'title'       => _x( 'Single', 'Template name' ),
+			'description' => __( 'The default template for displaying any single post or attachment.' ),
 		),
 		'page'           => array(
 			'title'       => _x( 'Page', 'Template name' ),